1. Field of the Invention
The present invention relates to an optical transmission system which Raman-amplifies, as well as transmits, signal light in an optical transmission line of a relay section, and to a Raman amplifying control unit which controls the Raman amplification of the signal light in the optical transmission system.
2. Related Background Arts
An optical fiber amplifier, which amplifies signal light so as to compensate the loss of signal light propagating through an optical transmission line in an optical transmission system, is equipped with an optical fiber for optical amplification and a means of supplying exciting light. Pump light of a given wavelength is supplied to the optical fiber for optical amplification by the exciting light supplying means, so that the signal light that is inputted to the optical fiber for optical amplification is amplified in the optical fiber for optical amplification and is outputted.
For such an optical fiber amplifier, there have been a Raman amplifier using Raman amplification phenomenon and a rare-earth-added optical fiber amplifier using an optical fiber for optical amplification in which a rare earth element is added to the light waveguide region thereof. The rare-earth-added optical fiber amplifier is modularized and is installed in a repeater and the like. On the other hand, the Raman amplifier uses an optical fiber for optical amplification (optical fiber for Raman amplification) in the optical transmission line or the part thereof in a relay section, and the optical amplification of the signal light is done therein. Therefore, if a Raman amplifier is used, not only can the effective loss (=transmission lossxe2x88x92Raman amplification gain) of an optical transmission line be reduced, but also the influence of an optical Kerr effect that is caused by the power of signal light that becomes too great in the various parts of the optical transmission line can be restrained.
In the optical transmission system that uses a Raman amplifier, it is important for the effective loss to be temporally stable in order to perform stable high-speed optical communication. Actually, however, there are cases where the effective loss varies because the Raman amplification gain of signal light by the Raman amplifier fluctuates due to the temporal variation of the transmission loss or the thermal or temporal variation of an exciting light source which outputs pump light for Raman amplification.
Such being the case, an invention for stabilizing the effective loss of an optical transmission line in an optical transmission system using a Raman amplifier is disclosed in Japanese Patent Application Laid-Open No. 3-13836. In this invention, an attempt is made in order to stabilize the effective loss by (1) introducing inspection light into the optical transmission line, (2) receiving the back-scattering light that arises when the inspection light propagates through an optical transmission line, (3) seeking, based on the results of the received back-scattering light, the loss distribution in a longitudinal direction of the optical transmission line, and (4) controlling, according to the loss distribution thus obtained, pump light for Raman amplification to be supplied into an optical transmission line.
In the above invention, only one set of inspection light introducing means for introducing inspection light into the optical transmission line, receiving means for receiving back-scattering light, and a means for controlling the pump light to be supplied to the optical transmission line are provided in a transmitting station or its vicinity. On the other hand, the exciting light supplying means for supplying pump light to the optical transmission line are provided at plural points of the optical transmission line from the transmitting station to the receiving station. Thus, the loss distribution of the optical transmission line from the transmitting station to the receiving station is measured by the inspection light introducing means and the back-scattering light receiving means that are provided in the transmitting station or its vicinity. All of the exciting light supplying means provided at plural points of the optical transmission line from the transmitting station to the receiving station are controlled altogether by the control means provided in the transmitting station or its vicinity.
An object of the present invention is to provide a Raman amplifying control unit that can stabilize the effective loss of signal light in an optical transmission line of an optical transmission system having a relay station between a transmitting station and a receiving station, and an optical transmission system using the same.
A Raman amplifying control unit according to the present invention, which can control an exciting light supplying means for supplying pump light into an optical transmission line so as to Raman-amplify signal light in the optical transmission line of an optical transmission system having at least one relay station, is equipped with (1) a means for outputting inspection light to inspect an optical transmission line and introducing the inspection light into the optical transmission line, (2) a means for receiving a back-scattering light which arises when the inspection light propagates through the optical transmission line, and (3) a means for controlling the supply of pump light to the optical transmission line from the exciting light supplying means, such control being made according to the results of inspecting the optical transmission line based on the results of receiving the back-scattering light by the back-scattering light receiving means. Moreover, the inspection light introducing means, the back-scattering light receiving means, and the control means are provided together with the exciting light supplying means in a station on the transmitting side or receiving side of the optical transmission line in the relay section of the optical transmission system.
Here, the term xe2x80x9crelay sectionxe2x80x9d means a section between a transmitting station and a relay station, between relay stations, or between a relay station and a receiving station. A transmitting station, a receiving station, and a relay station are generically referred to as a xe2x80x9cstationxe2x80x9d.
In an embodiment of a Raman amplifying control unit according to the present invention, wherein a station is a relay station, inspection light is introduced from the relay station into the optical transmission line on the upper stream side and the optical transmission line on the downstream side, respectively; in another embodiment, the inspection light introducing means introduces inspection light into the optical transmission line in a prescribed polarization state; in another embodiment, the light used as the inspection light has a plurality of wavelengths; in another embodiment, an inspection light source that can vary an output wavelength is used; and in another embodiment, the control means controls the supply of pump light according to the results of comparison between the received back-scattering light and a standard signal.
In an optional aspect of the Raman amplifying control unit of the present invention, the inspection light introducing means has a switching means.
In an embodiment of an optical transmission system according to the present invention, a Raman amplifying control unit controls the power of supplied pump light so that the power of signal light propagating through the optical transmission line on the downstream side does not exceed a pre-determined maximum value.
In another embodiment, a Raman amplifying control unit controls the power of the supplied pump light so that the power of signal light propagating through an optical transmission line on the upper stream side does not become lower than the pre-determined minimum value.
In an optional aspect of the Raman amplifying control unit of the present invention, an optical fiber amplifier is provided together with the Raman amplifying control unit in a station.
The above and further objects and novel features of the present invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.